Yeast
Yeast is the general name for single-celled fungi that ferment sugars into carbon dioxide and alcohol. It is the microbial engine behind beer, wine, mead, cider, sake, kvass, palm wine and kombucha. The term covers many species and strains, wild or cultivated, and does not by itself specify which organism or method is meant. In drinks, the yeast most often used is Saccharomyces cerevisiae, but lager yeast (S. pastorianus), Brettanomyces, Torulaspora and many wild species also play roles.
Usage in beverages
Yeast ferments the sugars in grain wort, grape and apple juice, honey water, palm sap, sweetened tea and bread mash. It produces alcohol, carbon dioxide and flavor compounds. In low- and no-alcohol drinks, fermentation is kept short or cool, or is paired with bacteria that convert ethanol to acids. Kvass, kombucha, fresh palm sap drinks and traditional ginger beer and sima work this way. Modern non-alcoholic beers may also use yeasts selected because they ferment only part of the available sugar.
In depth
Prehistoric and Ancient Near Eastern Brewing
The first brewers relied on yeasts that drifted in from the air, rode on fruit, or lingered in reused pots, without knowing these organisms existed. Grain-based fermented drinks appear very early in the archaeological record of the Levant and Mesopotamia. Egyptian art from roughly four thousand years ago depicts breweries alongside bakeries, showing how closely bread and beer shared the same leavening microbes. Researchers examining jars from several ancient sites in Israel, thought to have held beer and mead, managed to revive yeast colonies that had survived in the pottery for millennia. These finds are the first direct biological proof of yeast use in early societies. The English word itself traces back to an ancient root meaning to foam or bubble, a nod to the visible activity of fermentation.[1]
Wine, Mead and Cider in Western Eurasia
Grapes carry yeasts on their skins, so crushed fruit ferments almost on its own. Wine residues near present-day Tbilisi date to around 6000 BCE, and a dedicated winery in Armenia operated roughly two thousand years later. Wine culture then spread around the Mediterranean, was carried by monasteries through medieval Europe, and was later planted in the New World. Honey diluted with water ferments in much the same way, and mead may be humanity's oldest alcoholic drink. Ancient mead recipes simply left honeyed water in the sun, trusting the wild yeasts in the honey and on added fruit. Apple cider followed a similar pattern in northern Europe, relying on spontaneous fermentation in vats and casks. Today most winemakers and many mead and cider makers inoculate with chosen strains for predictable results. Mead makers often use white-wine yeasts or strains selected to preserve delicate honey aromas.[2]
Rice Drinks and Sake in East Asia
In China, fermented drinks made from rice, honey and fruit date to around 7000 BCE. Grain-based brewing later spread across East Asia, reaching Japan by roughly 500 BCE. East Asian rice drinks pair a mold with yeast. In sake, the koji mold Aspergillus oryzae breaks rice starch into sugar while yeast simultaneously turns that sugar into alcohol, a process known as multiple parallel fermentation. Temple brewers of the Muromachi period refined the starter mash, in which yeast is first multiplied under protective lactic acidity. Later methods such as kimoto, yamahai and the faster sokujo built on this step. In the early twentieth century, Japanese researchers isolated dedicated sake yeast strains. Mid-century strains that produced apple- and banana-like aromas helped create the fragrant ginjo style, and they prompted continuing research into aromatic yeasts.[3]
Kvass and the Low-Alcohol Bread Drinks of Northeastern Europe
In regions where grain was too scarce to make beer an everyday drink, people fermented stale rye bread or rye flour and malt into kvass. Kvass is a cloudy, sweet-sour drink usually holding well under one percent alcohol. Chroniclers record kvass being handed out in Kiev alongside mead in 988, at the celebration of Vladimir the Great's baptism. Households kept the drink going with a portion of the previous batch, a sourdough-style starter, or spontaneous fermentation. Commercial baker's yeast later became the norm, typically working for about half a day at room temperature. Lactic acid bacteria contribute the tang. Finnish kalja, Estonian kali and Lithuanian gira belong to the same family. Sweet Finnish sima, a lemon-flavored drink made with honey or brown sugar, is likewise lightly fermented with yeast, with raisins added to signal when it is ready.[4]
Palm Sap Across Africa, Asia and the Pacific
In the tropics, sap tapped from palm flowers or felled trunks begins fermenting almost immediately. Airborne yeasts and residues left in collecting containers start the process. Within a couple of hours the sweet white sap becomes a mildly alcoholic, aromatic drink. Acetic bacteria then gradually sour it toward vinegar, so it is best drunk fresh. Palm wine is central to weddings, funerals and ancestral libations among the Igbo, Yoruba and other West and Central African peoples. It is also sold as toddy in South Asia and as tuba in the Philippines and, by transpacific transmission, western Mexico. Where fermentation is deliberately stopped, the same sap becomes a non-alcoholic refreshment. Examples include neera in India, preserved with a little lime, and Mexico's tuba fresca, served with fruit and nuts. In Kerala, fermenting toddy has even stood in for baker's yeast to leaven rice hoppers.[5]
The Scientific Era and Pure Cultures
Yeast was first seen under a microscope in 1680, although its nature remained disputed. It was recognized as a fungus in 1837. Louis Pasteur's work in the 1850s showed that alcoholic fermentation is carried out by living yeast cells rather than a purely chemical reaction. Brewers came to distinguish top-fermenting ale yeast (S. cerevisiae) from the bottom-fermenting lager yeast (S. pastorianus) that thrives in the cold. Mechanical refrigeration then freed brewing and winemaking from the seasons. Cultivated yeast became a commercial product in the nineteenth century, sold first in liquid and cream forms and later as pressed blocks and granules. Most modern beer is now fermented with pure cultures. A few styles still depend on wild or unusual yeasts: Belgian lambic owes its character partly to Brettanomyces, and Torulaspora delbrueckii has been associated with Bavarian wheat beer.[1]
Kombucha and Today's Low- and No-Alcohol Ferments
Kombucha shows yeast working in partnership with bacteria. Sweetened tea is fermented by a gelatinous culture of yeasts and acetic acid bacteria, often called a mother. The yeasts include Saccharomyces, Brettanomyces, Zygosaccharomyces and others; they turn sugar into alcohol and carbon dioxide, and the bacteria convert much of that alcohol into tart acids. The drink likely originated in the Bohai Sea region of China. It spread to Russia and Central Europe by the nineteenth century, became a Japanese health fad in the 1970s, and boomed in the West in the early twenty-first century. Commercial versions are typically held below half a percent alcohol. Related cultures make water kefir and traditional ginger beer. Alongside these ferments, interest in lighter drinking has revived lightly fermented kvass and low-strength session meads. Brewers also use yeasts that ferment only part of the sugar in wort to make non-alcoholic beers that keep some of yeast's flavor contribution.[6]